In order to clean a solid surface such that the surface can again be coated such as, for example, to preserve metal against deterioration, or simply to degrease a solid surface, it has become common practice to use an abrasive blasting technique where the abrasive particles are propelled by high pressure fluid against the solid surface in order to dislodge previously applied coatings, scale, dirt, grease or other contaminants. An example of such a surface is the surface of aircraft.
Commercial airlines and military agencies spend large sums in periodically stripping or abrading paint and other coatings and contaminants from the exterior surfaces of modern aircraft. These surfaces comprise light weight aluminum or other metal alloys, composites, or plastics, which are relatively soft and from which paint or other coatings must be carefully removed to avoid excessive abrasion or chemical damage. Such damage may, in extreme cases, lead to mechanical failure.
Blasting media useful for removing coatings from sensitive metal and composite aircraft or like surfaces preferably meet the following criteria:
1. They should be relatively non-aggressive (Mohs hardness of about 2.0 to less than 4.0); PA1 2. They should be available in various particle size distributions for the treatment of different substrates; PA1 3. They should be free-flowing under high humidity conditions and throughout a broad range of air pressure and media flow rates; PA1 4. They should be water soluble and non-polluting to facilitate easy separation from the insoluble paints and resins stripped to facilitate waste disposal; and PA1 5. They should be non-corrosive to substrates and adjoining surfaces contacted by the blasting media.
Sand blasting, the principal means for cleaning or stripping hard durable surfaces such as granite or heavy steel plating, is too abrasive for removing coatings from sensitive substrates. Blasting techniques have therefore been used on sensitive substrates that employ materials softer than sand, such as plastic materials, walnut shell, and corn cobs. The disadvantages of these softer materials are that they are insoluble and therefore not easily disposable. Additionally, plastics are also expensive.
A blasting medium for removing coating from sensitive substrates containing water-soluble bicarbonate particles, preferably sodium bicarbonate, having average particle sizes of 100 to 500 microns, in admixture with a hydrophobic silica flow-anti-caking agent, has been developed by the owners of the present invention as taught in U.S. Pat. No. 5,160,547 to Kirschner et al. Although effective in removing coatings from substrates, the larger particle sizes of bicarbonate makes it a less efficient blasting medium than blasting medium employing compounds with small particle sizes, such as fine sand, which have better cutting efficiencies. Additionally, more water is needed to wash away the waste produced by bicarbonate-containing blasting medium than is generally used in commercial blasting systems because of the relatively low solubility of bicarbonate in water. Also, sodium bicarbonate particles are not sufficiently hard enough to effectively remove rust from a surface or cause profiling on the surface. Profiling is advantageous because the small indentations caused by the impact of abrasive particles on the surface increase the surface area of the article cleaned to provide an anchor for a new coat of paint.
Another blast medium suitable for removing coatings from sensitive metal and composite surfaces is a blast medium containing water-soluble crystalline sodium sulfate as disclosed in U.S. Pat. No. 5,112,406, also assigned to Church and Dwight Co., Inc. Water soluble sodium sulfate is an effective abrasive for removing coatings from such soft metals as light weight aluminum and the like. Water-soluble sodium sulfate when combined with flow aids does not cake and has an excellent shelf-life and is free-flowing. Sodium sulfate has a Mohs hardness almost the same as sodium bicarbonate. Thus, particles of sodium sulfate are not hard enough to effectively remove rust or cause profiling of a surface. Accordingly, there is a need for a more aggressive abrasive medium which can effectively remove coatings from sensitive surfaces.
A primary object of the present invention is a method for removing coatings from a relatively soft surface with a water-soluble abrasive medium.
Another object of the present invention is to provide for a water-soluble abrasive medium hard enough to cause profiling on a surface.
A further object of the present invention is to provide for a method and blast medium that removes rust from a surface.
Still yet another object of the present invention is to provide for a water-soluble blast media that is environmentally friendly.
Other objects and advantages will become apparent to those of skill in the art from the following description.